ESR meter

A standard (DC) milliohmmeter or multimeter cannot be used to measure ESR, because a steady direct current cannot be passed through the capacitor.

Aluminium electrolytic capacitors have a relatively high ESR that increases with age, heat, and ripple current; this can cause the equipment using them to malfunction.

When precision is required, measurements must be taken under appropriately specified conditions, because ESR varies with frequency, applied voltage, and temperature.

A general-purpose ESR meter operating with a fixed frequency and waveform will usually be unsuitable for precise laboratory measurements.

Circuit parameters are usually chosen to give meaningful results for capacitance from about one microfarad up, a range that covers typical aluminium capacitors whose ESR tends to become unacceptably high.

In a practical circuit, the ESR will be much lower than any other resistance in parallel with the capacitor, so it is not necessary to disconnect the component, and an in-circuit measurement can be made.

An ESR meter is more accurately described as a pulsed or high-frequency AC milliohmmeter (depending upon type), and it can be used to measure any low resistance.

The first major device to measure in-circuit ESR was based on Carl W. Vette's 1978 "US Patent #4216424: Method and apparatus for testing electrolytic capacitors".